1. Field of the Invention
This invention relates to a nulling optical bridge device and more specifically to a nulling optical bridge device which measures changes in reflectivity and/or transmissivity as a function of change in the relative power of two light beams.
2. Description of Related Art
It is necessary, for process characterization and control, to measure the characteristics of a semiconductor device, during various stages of processing.
For measuring temperature, contactless temperature monitoring is often the preferred method for taking these types of measurements. Contactless temperature measurement is, typically, accomplished by calibrated monitoring of the black-body radiation of the device (see for example U.S. Pat. Nos. 3,442,678; 3,462,224; 3,611,806; 3,654,809; 3,771,874; 4,020,095; 4,498,765). While this is a generally accepted method, it is not applicable under all conditions and is insensitive and subject to inaccuracies and error as for example when sample emissivity changes as a function of temperature or processing, or when emissivity is non-uniform over the sample surface or unknown, as when applied to silicon samples at temperatures of 500.degree. C. or less.
As a general scientific principle, it is known that a change of temperature in many substances, such as semiconductors and metals, is accompanied by a change in the optical reflectivity .DELTA.R and/or transmissivity of that substance. In addition, it is also recognized that the relative change in a sample's temperature .DELTA.T=(T-T.sub.o) with respect to some reference temperature T.sub.o (e.g., room temperature) can be determined by measuring the relative change in the optical reflectivity .DELTA.R=R(T)-R(T.sub.o) and/or transmissivity of that sample. However, these principles have, heretofore, been unable to be applied to contactless temperature measurement of devices comprised of a variety of materials, as the fractional change in reflectivity .DELTA.R/R and/or transmissivity with temperature for many of these materials is too small to be accurately measured. For example, for silicon at temperatures less than about 500.degree. C., .DELTA.R/R(T).perspectiveto.5.times.10.sup.-5 for a change in temperature of 1.degree. C. The accuracy of temperature measurements at these small changes becomes increasingly unreliable, as conventional light sources used for such measurements (for example, small lasers and/or incandescent lights) are subject to power variations in the range of 0.1-1.0%. These power variations make it impossible to obtain repeatable temperature resolution to the needed accuracy of about 0.1.degree. C. over a range of several hundred degrees centigrade.
One such device for the contactless measurement of temperature is shown in IBM Technical Disclosure Bulletin, Volume 28, No. 9, pp. 4004-4005, 1986, "In Situ, Contactless and Non-Destructive Measurement of the Temperature Variation Over the Surface of a Silicon Wafer" by D. Guidotti, S. I. Tan (inventors herein) and J. A. Van Vechten. However, the precision of the measurements made by this device are severely limited by the well recognized power fluctuation of the laser. These power fluctuations are known to introduce large noise components into the temperature measurement making the device unable to be used in applications requiring accurate temperature measurements.
It is therefore an object of the present invention to develop a device for the contactless measurement of changes in reflectivity and/or transmissivity of a material under processing.
It is therefore a further object of the present invention to develop a device for the contactless measurement of temperature of the material under processing with an accuracy of about 0.1.degree. C. over several hundred degrees .degree. C.
It is a still further object of the present invention to develop a device for the contactless measurement of ion implantation dose of a material under processing.